1. Field of the Invention
The invention relates generally to charging units and image forming apparatuses incorporating the units, and more specifically to a charging unit configured to reduce fluctuations of a charging gap, caused by the change in environmental conditions, a process cartridge, and an image forming apparatus incorporating the charging unit.
2. Discussion of the Background
The process of electrophotographic image formation is well known and is useful for both analog and digital copying and other reproduction techniques. Generally, the electrophotographic reproduction process is initiated by performing substantially uniform charging onto an image bearing member.
With respect to the charging, various devices and apparatuses have been proposed for creating a uniform electrostatic charge or charge potential on a photoconductive surface prior to the formation of latent images thereon.
Previously, corona generating devices such as scorotrons have been utilized to the charging, which is electrically biased to a high voltage potential.
Several problems have been associated with corona generating devices. For example, the use of very high voltages requires special insulation, inordinate maintenance of charging wires, and generates arcing caused by non-uniformities, and the contamination of corona wires. More importantly, it generates ozone and oxides of nitrogen, which eventually results in an adverse effect on the quality of the final output print produced by the reproduction apparatus.
As an alternative to corona generating devices, contact charging devices using roller or brush have recently been incorporated into various machines.
Since a charging roller operates to charge a photoreceptor while contacting thereto, the applied voltage is relatively low compared to non-contact chargers such as scorotrons, the amount of the above-mentioned generation of gaseous components such as ozone and NOx, can be reduced.
The contact charging devices, however, have several difficulties such as, for example, contamination by toner particles electrostatically adsorbed onto image bearing member (photoreceptor) and concomitant uneven charging, thereby resulting in an adverse effect on the life of charging unit.
In order to obviate the toner contamination, the method is previously known utilizing a film which is fixed to the ends of charging roller to form a minute gap between the roller and a photoreceptor. Alternatively, spacers are disposed to be fit into steps or grooves formed at the ends of charging roller to also form a minute gap. The range of gap width in these cases is disclosed ranging from 30 to 240 μm. (For example, Japanese Laid-Open Patent Applications No. 2001-194868, 2002-55508, 7-301973 and 8-202125.)
Although elastic members such as rubber or sponge are conventionally used for forming the charging roller, other materials also have been disclosed such as resinous materials (Japanese Laid-Open Patent Applications No. 2001-337515 and 2003-66693).
Also disclosed are several methods of forming the charging roller such as disposing rollers at the ends of charging roller to form a gap between a charging member and an image bearing member (Japanese Laid-Open Patent Applications No. 2001-312121 and 2000-206805); and forming an uppermost protective layer of a photoreceptor by dispersing inorganic particulates to improve abrasion resistance and mechanical strength of an organic photoconductor (Japanese Laid-Open Patent Application No. 8-339092), or particles of fluorocarbon resin to promote lubricating properties (Japanese Laid-Open Patent Application No. 11-218945).
As a further example, another charging roller as non-contact elastic member has been disclosed in Japanese Laid-Open Patent Application No. 2002-229307, in which the width of charging gap is found to fluctuate in the circumferential and axial directions ranging from 10 to 40 μm, and which the charging roller is operated under a DC bias potential superposed by AC bias which can be subjected to low voltage control.
Furthermore, a still another charging roller has been disclosed with the structure of spacers disposed at the ends of charging roller outside image forming region of an image bearing member, in which a charging member is formed of a resistive layer formed of fluorocarbon resin as the major component, an uppermost protective layer of a photoreceptor is dispersed with particulates of metal oxides or fluorocarbon resin, and the process of charging can be controlled for respective process units color by color (for example, Japanese Laid-Open Patent Applications No. 2002-251055, 2003-076101 and 8-184980).
With respect to the toner contamination described earlier, the degree of the contamination can be reduced by disposing the charging roller in non-contact arrangement compared with the contact manner.
However, there exists an upper limit for the width of the gap between charging roller and photoreceptor (which is hereinafter referred to as “charging gap”) primarily due to the properties of materials for forming the charging roller. Then, if the charging gap is larger exceeding a certain tolerance, abnormal discharge takes place and uneven image density emerges in the images produced.
In addition, since the proper value of charging bias varies depending on the charging gap, undesirable situation arises even in the case where the charging gap is within the certain tolerance: if a large deviation exits in the charging gap, the charging bias cannot be large enough to retain a suitable charging voltage where the charging gap is large, while discharge energy may be too large at the location where the charging gap is small.
As a result, the charging potential decreases due to the deficient charging bias for the former case, while filming of toner and additives onto photoreceptor and/or the increase in photoreceptor abrasion take place for the latter case due to the excessive discharge energy.
It is therefore preferable for the charging gap to be adjusted properly not only in terms of its average but also in deviation and fluctuation margin.
For example, in the case when the charging member is formed of rubber, the member suffers from a relatively large change in hardness and this result in a large change in charging gap. In order to obviate this difficulty, the methods have been disclosed in Japanese Laid-Open Patent Applications No. 2002-108059 and 2002-139893, in which several means for measuring charging gaps and then adjusting ones according to the change in environmental conditions.
In spite of the abovementioned disclosures, various difficulties remain yet to be solved.
For example, with respect to the proper adjustment of average and deviation described above, several descriptions have previously been found on the charging gap concerning its average and tolerance (the range of the charging gap for which uniform charging can be made). However, almost none has been found on the margin of fluctuation during the rotation of charging roller and photoreceptor, and the charging gap deviation in the longitudinal direction of the photoreceptor.
In the case when the charging member is formed of rubber, the difficulty persists in accuracy of cutting work of the rubber material and in relatively large change in hardness with temperature.
With respect to charging member formed of resinous materials, on the other hand, the materials have advantages for the ease of cutting work from its appropriate hardness. It has drawbacks due to the hardness, however, such as abrasion over the period of time when the resinous material is used as a thin film to form the gap forming member, and/or filtering out of adhesive agent from the edge portions and concomitant toner adhesion thereto.
When the photoreceptor formed of organic photoconductors, some damages may take place at the locations to which the charging forming member is brought into contact.
Such damage may be obviated by providing step portions at the ends of the gap charger to be disposed with an elastic material having a relatively large thickness as disclosed in Japanese Laid-Open Patent Application No. 2002-55508. In addition, with a similar structure providing circular groove formed in the step portion to be fit with a gap forming member, the noted damage can be obviated and slipping out of the gap forming member from the step portion can be prevented without using any adhesive agent.
Although some improvement of durability is expected by the use of the thick gap forming member, there persists a difficulty of relatively large thickness and concomitant increase in deviation of the charging gap.
In the disclosures, Japanese Laid-Open Patent Applications No. 2001-312121 and 2000-206805, the gap forming members each formed to be in contact with the surface of the photosensitive layer outside of image forming region of the photoreceptor, and degradation of the photoreceptor can be alleviated. However, a further gap between the charging layer and the gap forming member has to be provided to prevent the leakage there between. This may result in an undue increase in the length of core shaft of the charging roller and the size of the image forming apparatus as a whole.
In addition, although some adjustments of the charging gap according to the change in environmental conditions may be feasible to a certain extent by the methods disclosed in Japanese Laid-Open Patent Applications No. 2002-108059 and 2002-139893, the precision of the measurements is in the range of several tens of microns, and the measurements have to be carried out preferably under actual operating conditions. This may necessitate complex and costly mechanisms for measurements and adjustments.
It is an object of the present invention, therefore, to provide a charging unit configured to reduce fluctuations of charging gap, caused by the change in environmental conditions without additional means for measurement and adjustment and to achieve dependable durability at low cost, a process cartridge with the charging unit, and the methods for measuring and setting the charging gap.
It is a further object to provide an image forming apparatus incorporating the charging unit of the invention, capable of performing image duplication with excellent image qualities.